Habitat type modulates sharp body mass oscillations in cyclic common vole populations

Cyclic rodent populations exhibit pronounced changes in body mass associated with the population cycle phase, long-known as Chitty effect. Although Chitty effect is a common epiphenomenon in both America and Europe, there is still incomplete evidence about the generality of these patterns across the entire range of most species. Moreover, despite decades of research, the underlying factors driving Chitty effect remains poorly understood. Here, we examined the influence of intrinsic and extrinsic factors that may underlie observed patterns in vole size variation in the Iberian common vole Microtus arvalis asturianus. We weighed and measured 2816 adult voles that were captured during 6 trapping periods. Vole numbers and body mass showed strong period- and phase-related variation both in females and males, demonstrating marked Chitty effect in the studied population. Body mass of adult males correlated with body length, evidencing that heavier males are also structurally larger. Statistical models showed that probability of occurrence of large-sized vole (> 37 g) was significantly more likely in reproductive males, during increase and peak phases, and it was modulated by habitat, with crop fields and field margins between crops showing an increased likelihood. We suggest an effect of the habitat on vole body mass mediated by predation.


Bank vole (Myodes glareolus) France
Larger body weight is the result of a higher survival rate selecting for lower reproductive effort and a higher somatic allocation.Lower predation may contribute to the higher survival rate.

Several vole species Theoretical
Reproduction is suppressed in animals born or raised in the later part of the increase phase by environmental factors.The surplus energy will allow continuous growth.Animals grow to a larger size as a population enters the peak density phase, causing an increase in the average body mass. 7

Finland
Negative association between the mean body size of voles and weasel abundance was found in field observations.Laboratory experiments showed that smaller voles can escape from weasel predation, because they fit in holes too narrow for weasels.Size selective predation might be an important cause of the observed smaller voles in the decline phase of cycles.Although voles reach highest asymptotic weights in the peak phase and lowest asymptotes during the crash, initial growth rates were not significantly different.This suggests that voles attain larger body size during the peak phase as a result of growing for longer.12 Meadow vole (Microtus pennsilvanicus)

Virginia, USA
The large body masses observed in some voles were due more to long field lives than to unusually high rates of body growth.13 Prairie vole (Microtus ochrogaster) Illinois, USA Higher somatic growth in increase and peak phases of the population cycle.

Bank vole (Myodes glareolus) Finland
Voles from different cycle phases kept the same growth patterns when relocated to the same environment.Body size traits might be strongly influenced by heredity or early life conditions, rather than that of current juvenile and adult environmental conditions. in which medians of females were higher than males, likely due to a higher prevalence of pregnant females.

Figure S2 .
Figure S2.Boxplots for body mass of adult female voles trapped in each period (above) and for body mass ≥34g according to the cycle phases (below).Boxes represent 75% of data, and median is depicted as a black line.The length of the whiskers shows values from the 75 th percentile up to 1.5 × IQR (upper whisker) and from the 25 th percentile down to -1.5 × IQR (lower wisker).Pattern of body mass shown by females (plot above) was like males; yet somewhat difference can be observed in summer 2014 and 2016 in which medians of females were higher than males, likely due to a higher prevalence of pregnant females.

Table S1 .
Results of the GLMM explaining occurrence of vole body mass ≥37g (i.e.Chitty effect) including the variable cycle phases in which July 2016 was considered as increase phase (below table).

Table S2 .
Ranking of models

on the probability of occurrence of large vole (body mass
≥37g) (i.e.Chitty effect) ordered according to AICc and weight of the models."+": denotes a categorical variable included in the model; blank cell denotes that the variable is not included into model; the coefficients of continous variables and estimates of loglik, AICc, delta AICc and weight of each model are shown.Only models within 10 AICc units from the best model are shown.

Table S3 .
Summary of hypothesis proposed to explain Chitty effect in cyclic rodent populations.

growth and survival. They do not have an enhanced fitness under
conditions of rapid population growth and high densities.